1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus equipped with a zoom lens; the zoom lens may be suitable for use in a broadcasting television (TV) camera, a video camera, a digital still camera, a silver-halide film camera, or the like.
2. Description of the Related Art
A photographic optical system of an image pickup apparatus of a television camera or the like, typically includes a zoom lens in which high zoom ratio and high optical performance are desired. As a method of achieving a high zoom ratio, an internal extender method has been known. In the internal extender method, a focal length range of an entire zoom lens is displaced toward a long focal length side by inserting a focal length conversion optical system (extender) in apart of a relay unit which constitutes the zoom lens and which is stationary during zooming.
For example, U.S. Pat. No. 6,560,016 and U.S. Pat. No. 5,966,246 discuss a zoom lens which includes a first lens unit for focusing, a second lens unit for variation of magnification, a third lens unit for compensating variation of an image plane caused by the variation of magnification, and a fourth lens unit (relay unit) for image formation including a focal length conversion optical system. In the zoom lens discussed in U.S. Pat. No. 6,560,016 and U.S. Pat. No. 5,966,246, the fourth lens unit includes, in order from an object side to an image side, a first lens sub-unit, a focal length conversion optical system configured to be inserted into and removed from an optical path, and a second lens sub-unit. The focal length conversion optical system is inserted into an optical path so that the focal length range of the entire zoom lens is widened to have a longer focal length.
In an image pickup apparatus of a television camera or the like, a color image is obtained using a color separation optical system located on an image side. In order to obtain a satisfactory color image with small white shading over the entire image plane, it is important to control the incidence angle of a principal ray entering an image sensor to be substantially perpendicular over the entire image plane. To that end, the principal rays incident on the image sensor are collimated to be parallel to the optical axis over the entire image plane. This key property known as telecentricity can be obtained by careful design of multi-element lenses. Depending upon the optical spaces in which the principal rays exhibit this behavior, telecentricity may be classified into image-side, object-side, or dual telecentricity. A remarkable characteristic of telecentricity is constant magnification regardless of image and/or object location. For example, image-side telecentricity occurs when the system stop is placed at the image plane (rear focal plane) of the lens, which results in an exit pupil location at infinity. In this manner, a shift in the image plane does not affect image magnification. The same concept applies to object-side telecentricity.
Therefore, it is advantageous to configure a zoom lens to be in an image-side telecentric relation such that the position of an exit pupil is located as far away as possible from an image plane. In this manner, the same relation should be satisfied when a focal length conversion optical system is inserted into a relay unit to displace the focal length of the entire zoom lens toward a long focal length side. However, even when the position of an exit pupil is far away from an image plane, and favorable telecentric properties are obtained before the focal length conversion optical system is inserted into the optical path of the relay unit, if the position of the exit pupil becomes closer to the image plane when the focal length conversion optical system is inserted, color shading (in particular white shading) occurs, and the quality of a color image deteriorates.
In order to obtain a satisfactory color image with small white shading before and after the focal length conversion optical system is inserted into the relay unit, it is important to appropriately configure the lens configuration of the relay unit so that telecentricity is accurately maintained. Moreover, in order to realize miniaturization of the relay unit, it is important to appropriately set an F-number of the entire zoom lens.